Postextrasystolic potentiation does not distinguish ischaemic from stunned myocardium

Stunned myocardium and the attenuation of stunning by calcium antagonists

Myocardial "stunning" is characterized by a reversible postischemic contractile dysfunction despite full restoration of blood flow. The underlying mechanisms are not clearly understood. Inadequate energy supply and impaired sympathetic neurotransmission may have been excluded. Potential mechanisms, which are not mutually exclusive, may include damage to membranes and enzymes by free radicals, an increase in free cytosolic calcium during ischemia and reperfusion, and a lower calcium sensitivity of myofibrils. The equally pronounced increases in regional contractility in normal and stunned myocardium during postextrasystolic potentiation and the infusion of calcium or the calcium-sensitizing agent AR-L-57, however, suggest an unchanged calcium sensitivity in reperfused myocardium. Pretreatment with calcium antagonists before ischemia attenuates myocardial stunning. This effect is probably related to a lessened myocardial calcium overload during early ischemia. The potential benefit of treatment with calcium antagonists after reperfusion is established remains controversial.

Cellular and mitochondrial energy metabolism in the stunned myocardium

The influence of cardiac stunning on the oxidation of fatty acids and the oxidative phosphorylation in mitochondria was investigated. Rat hearts were perfused for 15 min according to the working mode with a Krebs-Henseleit buffer containing glucose (11 mM). The hearts were then maintained in normoxic conditions (C group) or subjected to a 15-min global no-flow normothermic ischemia followed by a 30-min reperfusion (R group). Throughout the perfusion, the aortic and coronary flows, and the heart rate and oxygen consumption were monitored. At the end of the perfusion procedure, a bolus of 1-14C palmitate was injected in the coronary arterial bed to evaluate the fatty acid oxidation. Two sub-populations of mitochondria were isolated from each heart by either mechanical (ME mitochondria) or enzymic (EE mitochondria) extraction and their respiration properties were evaluated. Furthermore, the mitochondrial energy production (ATP and creatine phosphate) was assessed. During ischemia, the aortic flow was suppressed and recovered only to approximately 50% of the preischemic value during reperfusion. This mechanical stunning was associated with an important reduction of the stroke volume (-37%, p < 0.01) and a slight decrease in heart rate (-20%, p < 0.001). At the end of reperfusion, the beta-oxidation rate constituted 55 +/- 1.7% of the cell palmitate and was similar to that assessed in the C group. The oxygen consumption was decreased to 216 +/- 31.0 microL O2/min/gww and the venous O2 concentration increased to 5.1 +/- 0.572 microL O2/mL (instead of 2.9 +/- 0.342 microL O2/mL in the C group), although due to large SD, only the latter was statistically significant. A decrease in metabolic efficiency (42 +/- 14.4 vs 106 +/- 16.8 mL/microL O2 in the C group) and an increase in palmitate oxidation to oxygen consumption ratio (77 +/- 10.1 vs 47.6 +/- 4.25% beta-oxidized palmitate/microL O2 in the C group) were observed. This increased fatty acid contribution in the oxidation metabolism could be responsible for some oxygen wasting and could contribute to decrease the energy available for the contraction despite the normal cardiac oxygen uptake. Furthermore, the respiration parameters of the mitochondria were similar in the C and R groups when glutamate (20 mM) or palmitoylcarnitine (25 microM) were used as substrate. ME mitochondria of R group displayed a reduced rate of ATP production (118 +/- 29.5 vs 180 +/- 14.5 nmoles/min/mg proteins in the C group) without altered creatine phosphate production. The presence of calcium in the medium (10(-5) M) provoked a decrease in ATP production.(ABSTRACT TRUNCATED AT 400 WORDS)

Recruitment of an inotropic reserve in moderately ischemic myocardium at the expense of metabolic recovery. A model of short-term hibernation

The regional, functional as well as metabolic consequences of inotropic stimulation on myocardium subjected to prolonged moderate ischemia were investigated. In 35 enflurane-anesthetized swine the left anterior descending coronary artery was cannulated and perfused at constant flow. The vein paralleling the left anterior descending coronary artery was cannulated for measurement of lactate and oxygen content. Transmural biopsies from the anterior myocardium were taken for the measurement of ATP, creatine phosphate, and glycogen. After control measurements, flow was adjusted to reduce regional contractile function (expressed as a work index, determined by sonomicrometry) by approximately 50%. After either 5, 25, 40, or 85 minutes of moderate ischemia, dobutamine was infused for 5 minutes into the ischemic region. In a separate group of five swine also subjected to 85 minutes of ischemia followed by infusion of dobutamine and 2 hours of reperfusion, triphenyltetrazolium chloride staining and light microscopy were used to identify infarcted tissue. Moderate ischemia (regional myocardial blood flow, 0.21 +/- 0.07 ml.min-1.g-1, determined by radiolabeled microspheres) was associated with a reduction of creatine phosphate after 5 minutes (from 9.35 +/- 2.54 to 6.43 +/- 1.06 mumol/g wet wt, p less than 0.05) and a further reduction after 25 minutes (3.18 +/- 0.69 mumol/g wet wt, p less than 0.05). Thereafter, creatine phosphate recovered despite continued ischemia (after 40 minutes, 4.95 +/- 1.37 mumol/g wet wt; after 85 minutes, 5.78 +/- 2.27 mumol/g wet wt). Lactate consumption during control conditions was reversed to production after 5 minutes of ischemia, which moderated during more prolonged ischemia. Without changing regional myocardial blood flow, infusion of dobutamine increased the work index significantly at any time point but also caused worsening of metabolic markers of ischemia. Nevertheless, even after 85 minutes of ischemia followed by the infusion of dobutamine and 2 hours of reperfusion, there was no evidence of necrosis. This experimental model provides a means of characterizing the mechanisms of short-term hibernation.